Evaluation points of network equipment are summed up here.
What should be concretely considered for a network equipment?
The essential points for the equipment having two interfaces are as follows:

Does it transfer something between interfaces?

If it transfers, what layer is transferred?

What is the protocol transferred?

Where is the direction transferred?

What is the condition for the transfer?

What technology is used for the transfer?

When considering with a focus on the above points, it is easy to understand characteristic of individual network equipment.

Figure A-7-1 Concept of the equipment having two interfaces

To understand functions of recent network equipment (equipment having two or more interfaces in this case), the following three points have to be paid attention.

1

In network equipment, "data to be transferred" and "data to be given as an independent host" are different, so verification is necessary respectively.
For example, an equipment which handles only the physical layer can check or configure the status by SNMP (which monitors and controls communication equipment via a network) or Web, in some cases. An IP address is assigned to such a equipment. However, it cannot necessarily transfer data of the network layer.

2

It can be considered that wireless LAN stations have interfaces on the wireless LAN side. (In this case, data transfer occurs between Ether side and wireless LAN side.)
In wireless LAN, misunderstanding is generated comparatively easily because the interface is invisible. In particular, caution is needed for what the layer transferred is.

3

A lot of equipment provides various services as well as the transfer.
For example, there are DHCP Server, Name Server and so on. However, the type of service provided in each interface has to be investigated. If it is unclear, unnecessary (irregular in many cases) services are provided to the third party unknowingly. That causes a situation where a part of the network is shut down or the traffic gets heavy by the confusion.
When some kind of service is provided, the interface for which the service is provided has to be made clear.

The above 1 to 3 cannot be often isolated at all. Therefore, in particular, caution is needed when using a recent equipment for the broadband. (Because the specification does not describe them.)

The arrangement of functions based on the OSI 7-layer model or TCP/IP layer model allows to understand correctly the state of network equipment having complex functions. Moreover, the correct understanding of roles of individual equipment in a network is needed to design the network correctly. Their foundation is the OSI 7-layer model and TCP/IP layer model.

Even if a line is connected, a network does not operate. A network does not operate unless the functions and performances of equipment to be deployed are understood correctly and the appropriate deployment is designed.